As 3,3′,4,4′-biphenyltetracarboxylic dianhydride-based polyimides, which yield heat-resistant polyimides, there are known polyimides comprising 3,3′,4,4′-biphenyltetracarboxylic dianhydride and para-phenylenediamine as the starting acid dianhydride component and the starting diamine component, respectively, and these yield polyimides with low linear expansion coefficients and large elastic moduli.
Films made of such polyimides have excellent thermal properties and electrical properties and are therefore widely employed in electronic devices. However, a high adhesive strength cannot be obtained with adhesives ordinarily used in the field of electronics, and laminated bodies with high peel strengths cannot be obtained by forming metal layers by metal vapor deposition or sputtering.
A film composed of such a polyimide has low saturated water absorptivity and a low hygroscopic expansion coefficient and therefore has the advantage of dimensional stability against environmental changes. However, because of its extremely low moisture absorption rate, if it is exposed to high temperature in subsequent soldering steps when it is used as a cover lay film or the like, the trace moisture remaining on the base cannot parmeate the cover lay film and causes foaming or peeling at the adhesive interface.
Attempts have been made to improve the low adhesion of such polyimide films. For example, polyimide films with improved adhesion are known which comprise 0.02-1 wt % of compounds of tin, bismuth or antimony (Japanese Unexamined Patent Publication HEI No. 4-261466, Japanese Unexamined Patent Publication HEI No. 6-299883, Japanese Patent Public Inspection HEI No. 7-503984). However, such polyimide films potentially exhibit poorer electrical properties such as electrical insulation. Also known are techniques for improving the adhesion of polyimide films by plasma discharge treatment (Japanese Unexamined Patent Publication SHO No. 59-86634, Japanese Unexamined Patent Publication HEI No. 2-134241). However, discharge treatment often has an insufficient effect on improving the polyimide film adhesion, and productivity is low because of the requirement for complex post-treatment steps.
It has also been attempted to improve the gas permeability of such polyimide films. For example, polyimide films with improved gas permeability are known which use polyimides obtained from diamines having bulky trimethylsilyl groups bonded to the aromatic rings (Japanese Unexamined Patent Publication No. 2004-224889). Also, it has been attempted to use polyimides with bulky CF3 groups on the aromatic rings to increase the distance between molecular chains and improve the gas permeability (W. J. Koros, G. K. Fleming, Journal of Membrane Science, Holland, 1993, Vol. 83, p. 1-80). However, such starting materials are expensive and cannot be easily applied to industrial use.